Re: [PATCH 4/4] add ksm kernel shared memory driver.

[Anthony Liguori <anthony@codemonkey.ws>]

Andrea Arcangeli wrote:
> On Tue, Mar 31, 2009 at 10:54:57AM -0500, Anthony Liguori wrote:
>   
>> You can still disable ksm and simply return ENOSYS for the MADV_ flag.  You 
>>     
>
> -EINVAL if something, -ENOSYS would tell userland that it shall stop
> trying to use madvise, including the other MADV_ too.
>
>   
>> could even keep it as a module if you liked by separating the madvise bits 
>> from the ksm bits.  The madvise() bits could just provide the tracking 
>> infrastructure for determine which vmas were currently marked as sharable.
>> You could then have ksm as loadable module that consumed that interface to 
>> then perform scanning.
>>     
>
> What's the point of making ksm a module if one has part of ksm code
> loaded in the kernel and not being possible to avoid compiling in?
> People that says KSM=N in their .config (like embedded running with 1M
> of ram), don't want that tracking overhead compiled into the kernel.
>   

You have two things here.  CONFIG_MEM_SHARABLE and CONFIG_KSM.  
CONFIG_MEM_SHARABLE cannot be a module. If it's set to =n, then 
madvise(MADV_SHARABLE) == -ENOSYS.

If CONFIG_MEM_SHARABLE=y, then madvise(MADV_SHARABLE) will keep track of 
all sharable memory regions.  Independently of that, CONFIG_KSM can be 
set to n,m,y.  It depends on CONFIG_MEM_SHARABLE and when it's loaded, 
it consumes the list of sharable vmas.

But honestly, CONFIG_MEM_SHARABLE shouldn't a lot of code so I don't see 
why you'd even need to make it configable.

>> A number of MADV_ flags are Linux specific (like 
>> MADV_DOFORK/MADV_DONTFORK).
>>     
>
> But those aren't kernel module related, so they're in line with the
> standard ones and could be adapted by other OS.
>
> KSM is not a core VM functionality, madvise is a core VM
> functionality, so I don't see fit. KSM as ioctl or KSM creating
> /proc/<pid>/ksm when loaded, sounds fine to me instead. If open of
> either one fails, application won't register in. It's up to you to
> choose KSM=M/N, if you want it as core functionality just build as
> KSM=Y but leave the option to others to save memory.
>   

The ioctl() interface is quite bad for what you're doing.  You're 
telling the kernel extra information about a VA range in userspace.  
That's what madvise is for.  You're tweaking simple read/write values of 
kernel infrastructure.  That's what sysfs is for.

Regards,

Anthony Liguori